In general, dielectric ceramic materials are reduced to semiconductors by firing under a low oxygen partial pressure in, for example, a neutral or reducing atmosphere. Accordingly, internal electrodes of a monolithic ceramic capacitor comprising such a dielectric ceramic material have to be formed of a conductive material, such as palladium and platinum, that is not oxidized by firing under a high oxygen partial pressure nor is melted at the sintering temperature of the dielectric ceramic material. This makes it difficult to reduce the price and increase the capacitance of the monolithic ceramic capacitor.
The use of base metals, such as nickel and copper, for the internal electrode is expected to overcome the disadvantages. Unfortunately, these base metals used as a conductive material of the internal electrodes are undesirably oxidized by firing under a high oxygen partial pressure.
Accordingly, a dielectric ceramic material is required which is not changed to semiconductor by firing in a neutral or reducing atmosphere with a low oxygen partial pressure and which provides excellent dielectric characteristics if these base metals are used.
As nonreducing dielectric ceramics, particularly strontium titanate-based nonreducing dielectric ceramics, satisfying these requirements, compositions expressed by the formula (Sr1-xCax)(Ti1-yZry)O3 have been proposed (for example, in Japanese Unexamined Patent Application Publication Nos. 2000-53466 and 63-224106).
Since these dielectric ceramic materials are not changed into semiconductors by firing in a reducing atmosphere, base metals, such as nickel and copper, can be used as a conductive material for the internal electrodes of monolithic ceramic capacitors.
In addition, another strontium titanate-based nonreducing dielectric ceramic has been proposed (in, for example, Japanese Unexamined Patent Application Publication No. 2001-351828) which is expressed by the formula (Ca, Sr, Ba, Mg)(Zn, Ti, Mn, Ni, Hf)O3. This type of nonreducing dielectric ceramic advantageously exhibits small variations in characteristics and a high reliability.
Unfortunately, the relative dielectric constant of the strontium titanate-based nonreducing dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 2000-53466 is as low as less than 150.
As for the strontium titanate-based nonreducing dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 63-224106, if the thickness of dielectric ceramic layers formed of this ceramic between the internal electrodes is reduced to achieve a miniaturized high-capacitance monolithic ceramic capacitor, the reliability for high temperature-loading is disadvantageously reduced.
Also, the relative dielectric constant of the strontium titanate-based nonreducing dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 2001-351828 is disadvantageously as low as less than 100.
Accordingly, an object of the present invention is to provide a strontium titanate-based nonreducing dielectric ceramic exhibiting a high relative dielectric constant of 150 or more and a high reliability for high temperature loading, and a method for manufacturing the same.
Another object of the present invention is to provide a monolithic ceramic capacitor comprising the nonreducing dielectric ceramic.